How do I change the transparency of an object without changing the material (Cycles)?
Using pass index

You can use the object 'pass index' from the 'object info' input node.

Pass index is an integer. But you can divide it by a given 'scale' value to have a progressive transparency:



So that:



Note: pass index will animate (keyframes) in render mode only.

Using object proximity

Another way can be testing the proximity of an object/empty to the mesh:



In this case, the node setting can be using object texture coordinates from both the mesh itself and an empty.

Then we take the difference and test it on X, Y (eventually Z), then multiply (will make like a ponderated logical 'and' then divide by a tuning value).